In order to treat cancer with radiation, it is necessary to deliver the dose prescribed to the target volume, while minimizing the dose to other areas.
Many mechanical configurations of radiation therapy machines and the associated radiation sources have been developed since Roentgen discovered X-Rays. Modern radiation therapy systems use relatively high energy beams of radiation from radioactive isotopes or electron beam X-Ray generators. The X-Ray generators can employ either high voltage direct current or RF driven linear accelerators (LINACs). A mainstream radiation therapy system uses a LINAC to generate an electron beam with between 4 and 22 MeV of energy at low current. The electron beam strikes a high-atomic number target, typically tungsten, and generates penetrating x-rays. The beam is shaped and delivered to the target volume from one or more directions. The overlapping dose at the target volume is generally higher than the dose at the surface from any one delivery angle. The skin is sensitive to radiation, so it is desirable to limit the skin dose to minimize complications. If more fixed beam angles or continuous rotation are used, the surface dose can be spread out more and minimized with respect to the dose delivered to the target volume. It is also desirable to minimize the stray radiation dose to the rest of the patient. Low levels of radiation delivered to a large volume can trigger cancer growth in patients that survive the primary disease for a long time. A significant fraction of all radiation therapy treatments is employed to treat breast cancer with very good success. A typical general purpose radiation therapy system is designed to treat virtually all anatomical sites with some trade-offs being made in the design in order to make a universally applicable machine.